The present invention relates generally to providing a standard for calibration of optical signals and more particularly to calibrating wavelength division multiplexing of optical signals in fiber optics communications.
As the use of optical devices for communications and other applications increases, the need for accurate calibration of optical devices such as optical spectrum analyzers and tunable lasers has grown rapidly.
One such application that is rapidly growing in importance is that of Wavelength Division Multiplexing. Wavelength Division Multiplexing (WDM) is a way of increasing the capacity of an optical fiber by simultaneously operating at more than one wavelength within a single optical fiber. Multiple optical signals of different wavelength are transmitted in the same direction over one strand of fiber, and the signals are later separated by wavelength at the distant end. In order to establish some standards for WDM operations, the International Telecommunication Union (ITU) has proposed successive systems of standardized frequencies to be used as channels for optical telecommunications, with each system incorporating more and more channels, usually with smaller and smaller wavelength separation between the channels. This system of channels is spoken of as the xe2x80x9cITU gridxe2x80x9d and currently includes 80 channels utilizing a wavelength range centered around 1550 nm (193,300 GHZ ) with a channel spacing of approximately 0.4 nm (50 GHZ). There has also been a system proposed which uses channel spacing of 25 GHZ (0.2 nm). It will be easily apparent that the smaller the spacing is between channels, the more sensitive a multiplexed channel system will be to fluctuations that will cause the channel frequencies to drift away from the target grid frequencies. In a system where the frequency spacing is 100 GHz, the variation from the grid frequency of 0.01 nm (1.2 GHz) would be undesirable, but perhaps may not be disastrous. In a 50 GHz system, a 0.01 nm variation would impair the performance of the system, and in a 25 GHz system, such a deviation would likely be disabling.
Various optical devices such as wavelength meters, optical spectrum analyzers and tunable lasers all require a wavelength reference of the ITU grid in order to provide users with properly calibrated data. A random survey of calibrations conducted by the Optical Society of America indicates that the calibrations can be off from the ITU grid by as much as 0.3 nm (38 GHz). In addition, many devices have scan dependent calibration errors, meaning that calibration cannot be performed at only one or two data points, and it may not be sufficient to have one sweep rate and wavelength calibrated. Instead it may be necessary to have each parameter calibrated prior to measurement. In wavelength meter applications, the accuracy of the wavelength meter is generally unsatisfactory for 50 GHz measurements, which demand 1 xcexcm accuracy or better. For tunable laser applications, the tunable laser should be stabilized at select ITU wavelengths. For all these applications a highly stable and accurate frequency/wavelength reference standard which is aligned with the ITU grid is highly desirable. Recently interference filters have been proposed as a solution to providing such a standard.
An interference filter is a type of tool that is often used to separate multiple wavelengths of light that are included in a beam of light. A Fabry-Perot Interferometer is one type of interference filter that is often used for wavelength filtering and separation. Interference filters operate by providing a pair of mirrored surfaces with a cavity formed between them. Incident light undergoes multiple reflections between the mirrored surfaces, which typically reflect greater than 95% of the light at each surface. The incident and reflected wave interfere with each other constructively or destructively depending on their phase relationship. Where there is no phase difference between successive waves, constructive interference is produced and a maximum is produced in the transmission portion. Where the waves are 180 degrees out of phase, destructive interference occurs and a minimum is transmitted. A maximum occurs when the round trip optical path is some integer multiple of whole wavelengths, and also depends on the thickness of the cavity (d), the index of refraction of the cavity material (n), and the angle of incidence (xcex8), which are related by the formula:
2 d n cos xcex8=m xcex, 
where m is an integer, often termed the order number and xcex is the wavelength of the light. The parallel rays of each wavelength are often focused by a lens in order to produce a familiar ring pattern. The result is a series of transmission peaks of separated wavelength. The separation distance between adjacent peaks is equally spaced when plotted with respect to inverse wavelength, and is called the Free Spectral Range (FSR).
Etalons are special Fabry-Perot interferometers which have fixed spacing between the reflective surfaces, thus the thickness of the cavity d is therefore not subject to direct parallel variation. However, the etalon may be tilted, changing the angle of the etalon relative to the angle of incidence of the light beam, which thus increases the optical path length. This allows the etalon to be xe2x80x9ctunedxe2x80x9d over a limited range to alter the peak transmission wavelengths.
An ITU grid 2 with 100 GHz frequency spacing 6 is shown in FIG. 1 with the fringe order pattern 4 from a Fabry-Perot interferometer using an etalon of appropriate parameters superimposed on the grid 2, the spectral lines appearing as sharp notches 8.
FIG. 2 shows an etalon as used in the prior art. The incident light strikes the etalon which has been tuned at angle xcex8, thereby increasing the optical path length n to angle tune the etalon. However, angle tuning of an etalon introduces other problems caused by the insertion loss due to the variation in angle. Besides the difficulties of producing very tiny variations in angle, when the etalon is tuned at a small angle, the output beam can become oblong in shape, with non-uniform beam intensity distribution. As this angle increases, this effect becomes more pronounced. When optics are used to collect the output light from the etalon, a large insertion loss variation is often seen. This variation is typically from 1-4 dB.
The beam also xe2x80x9cwalks-offxe2x80x9d from its original position due to the refraction effect of entering a medium at an angle not normal to the surface. This walk-off is of course undesirable in any system where precise positioning of the beam is important.
The variation of etalon insertion loss also commonly causes the operating point of the output spectrum to shift by as much as 10 pm (0.01 nm=1.2 GHz). As discussed above, errors of this magnitude can seriously interfere with operation of systems which use 25 Ghz frequency separations and even with 50 GHz systems.
Thus, there is a great need for a etalon which is usable in a multi-channel wavelength locking system which does not produce such large variations in insertion loss, beam quality, and wavelength shift.
Accordingly, it is an object of the present invention provide an ITU frequency/wavelength reference which produces smaller variations in insertion loss.
Another object of the present invention is to provide an ITU frequency/wavelength reference which produces smaller variations in beam quality.
An additional object of the present invention is to provide an ITU frequency/wavelength reference which produces less walk-off of the beam.
Yet another object of the present invention is to provide an ITU frequency/wavelength reference which can be used with wavelengths which are separated by as little as 25 GHz or less.
A further object of the present invention is to provide an ITU frequency/wavelength reference which has reduced manufacturing costs due to more relaxed dimensional tolerances in the parts.
A yet further object of the present invention is to provide an ITU frequency/wavelength reference which can be used as a calibration device for various applications such as wavelength meters or optical spectrum analyzers.
Briefly, one preferred embodiment of the present invention is a wavelength reference which includes one or more gas-tunable etalons. Each etalon has first and second reflective surfaces, making a reflecting surface pair. Each reflecting surface pair surrounds a cavity, the cavity being filled with a gas-tunable medium having a variable optical index of refraction. The etalons produce equally-spaced spectral lines which are variable in response to changes in the gas properties such as gas pressure or composition. The spectral lines are tuned to align to an external wavelength standard, preferably an ITU reference grid. The properties of the gas-tunable medium are then fixed, preferably by sealing an enclosure which surrounds the etalons, so that they act as a wavelength reference.
The second reflecting surfaces of the etalons can be totally reflective, or in an alternate embodiment, can be at least partially transmissive.
An advantage of the present invention is that it provides improved ITU setting repeatability for systems in which it is used.
Another advantage of the present invention is that it provides improved temperature stability.
And another advantage of the present invention is that it produces very small variations in insertion loss and beam quality.
A further advantage of the present invention is that it can be used with wavelengths which are separated by as little as 25 GHz or less.
A yet further advantage is that can provide a tunable etalon which has reduced manufacturing costs due to more relaxed dimensional tolerances in the parts.
Yet another advantage of the present invention is that it is usable as a calibration device with a number of OSA devices, such as wavelength meters, etc.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.